Phosgene is the chemical
compound with the formula
Cl2CO. This colourless gas gained infamy as a chemical
weapon during World War
I, but it is also a valued industrial reagent and building
block in organic
synthesis. In low concentrations, its odor resembles freshly
cut hay or grass. In addition to its industrial production, small
amounts occur naturally from the breakdown of chlorinated compounds
and the combustion of
chlorine-containing
organic
compounds. Approximately 5000 Mg were produced in 1989.

Structure and basic properties

Phosgene is a planar
molecule as predicted by VSEPR
theory. The C=O distance is 1.18 Å, the
C---Cl distance is 1.74 Å and the Cl---C---Cl angle is 111.8°. It
is one of the simplest acid
chlorides, being formally derived from carbonic acid.

Production

Industrially, phosgene is produced by passing
purified carbon
monoxide and chlorine gas through a bed of
porous activated
carbon, which serves as a catalyst. The equation is
described as follows: Although much less dangerous than most other
chemical weapons (e.g. mustard gas),
phosgene is still regarded as a viable chemical warfare agent
because it is so easy to manufacture when compared to the
production requirements of more technically advanced chemical
weapons such as the first-generation nerve agenttabun.

Adventitious occurrence

Upon ultraviolet radiation in the
presence of oxygen,
chloroform slowly
converts into phosgene via a radical
reaction. To suppress this photodegradation, chloroform is
often stored in brown-tinted glass containers. Chlorinated
compounds used to clean oils off of metals may also react under the
UV created in a welding arc to produce phosgene.

Synthesis of acid chlorides

Such
acid chlorides react with amines and alcohols to give,
respectively, amides and esters, which are commonly used
intermediates. Thionyl
chloride is more commonly and more safely employed for this
application. A specific application for phosgene is the production
of chloroformic esters:

Chemical warfare

further
Use of poison gas in World War I Phosgene was stockpiled by
various countries as part of their military arsenals until well
after World War
II. The United States began disposing of its stockpiles in
1969. Even
before then, the importance of phosgene as a weapon had declined as
the more lethal nerve agents
entered stockpiles. On August 24th, 2007, vials of purported
phosgene were found near the United Nations headquarters in New
York City, where the sample had been forgotten after being
retrieved from Iraq in 1996. The FBI helped remove the chemicals
and there was no danger. Preliminary sampling indicates a
non-threatening agent (industrial solvent) as first reported by The
Daily News (NYTimes.com, September 5, 2007).

Bhopal disaster

further Bhopal
disaster In 1984 in Bhopal, India 43 tonnes of methylisocyanate was
accidentally released, killing or injuring tens of thousands of
people. One of the components of the leak was probably
phosgene.

Safety

Phosgene is an insidious poison as the odor may not
be noticed and symptoms may be slow to appear. Phosgene can be
detected at 0.4 ppm, which is 4x the Threshold
Limit Value. Its high toxicity arises, not from hydrogen
chloride released by hydrolysis, but by the action of the phosgene
on the proteins in the pulmonary alveoli. The alveoli are the
site of gas exchange, and their damage disrupts the blood – air
barrier and causing suffocation. Phosgene detection badges are worn
by those at risk of exposure.